1. Technical Field
The present invention relates to a substrate with a silicon carbide film, a method for producing a substrate with a silicon carbide film, and a semiconductor device.
2. Related Art
Silicon carbide (SiC) is a wide band gap semiconductor having a band gap (2.36 to 3.23 eV) two or more times wider than that of silicon (Si), and has attracted attention as a material for a high withstand voltage device recently.
However, since the crystal formation temperature of SiC is high unlike Si, it is difficult to form a single crystal ingot by a pulling method from a liquid phase in the same manner as a Si substrate. Therefore, a method in which a single crystal ingot of SiC is formed by sublimation has been proposed. However, it is very difficult to form a substrate having a large diameter and few crystal defects by sublimation. On the other hand, among SiC crystals, cubic SiC (3C-SiC) can be formed at a relatively low temperature, and therefore, a method in which cubic SiC is epitaxially grown directly on a Si substrate has been proposed.
As one method for producing a SiC substrate using this epitaxial growth, a heteroepitaxial technique in which 3C-SiC is stacked on a Si substrate in a gas phase has been studied. However, the lattice constants of Si and 3C-SiC are 5.43 Å and 4.36 Å, respectively, and there is an about 20% difference. Further, the thermal expansion coefficient of Si is 2.55×10−6 K−1, and the thermal expansion coefficient of 3C-SiC is 2.77×10−6 K−1, and there is an about 8% difference. Therefore, it is very difficult to obtain a high-quality epitaxial film (3C-SiC film) with few crystal defects from these materials. Further, these differences in the lattice constant and the thermal expansion coefficient result in generating large stress in the 3C-SiC film, and as a result, a problem of warpage of a wafer also occurs.
The crystal defects as described above are mainly so-called twin defects and anti-phase boundary (APB) defects. As a method for reducing these crystal defects, for example, in JP-A-11-181567 discloses a method for effectively reducing crystal defects by forming an oxide film or the like on a growth base substrate, providing an epitaxial growth region using this oxide film or the like as a mask, and setting the thickness T of the oxide film or the like with respect to the width W1 of an opening of the growth region to tan (54.6°)×W1 (tan (54.6°) times) or more. In this case, assuming that the width W1 of the opening is set to, for example, about 0.5 μm, the oxide film or the like to be used as the mask needs to have a thickness T of 0.7 μm or more.
By using a currently available process technique, the difficulty level of a pattern forming process is not so high, however, it is necessary to form a patterned oxide film or the like with high accuracy at a relatively high aspect ratio. Further, the thickness of the oxide film or the like is relatively large, and therefore, a problem that the SiC substrate is warped or the like due to film stress generated in the oxide film or the like occurs.